They way I've understood it is that if you set the authentication pattern to Windows, then that means NTLM. If you're not using "Windows" then you can't connect using NTLM authentication and you'll have to do it some other way. This is an excerpt from the help page on Virtual proxies (https://help.qlik.com/en-US/sense/September2017/Subsystems/ManagementConsole/Content/edit-virtual-proxy.htm):
"Windows authentication pattern - The chosen authentication pattern for logging in. If the User-Agent header contains the Windows authentication pattern string, Windows authentication is used. If there is no matching string, form authentication is used."